267 



FAM. XV. NOSTOCHINE.E. 



The Nostochinece form one of the most natural and beauti- 

 ful of the families of freshwater Alga. The filaments are 

 simple, of uniform diameter, elegantly moniliform, resembling 

 strings of pearls, in the highest degree flexible, and of ex- 

 ceeding lubricity. The species of which it is composed 

 naturally arrange themselves into two divisions : in the one 

 the filaments are free, and in the other imbedded in a mucous 

 matrix, which sometimes assumes a definite form. 



At intervals, in the course of the filaments, are observed 

 cells larger than those which compose the thread itself: these, 

 in the genus Anabaina are more or less of an oval or elongated 

 form ; while in the genus Nostoc they are exactly spherical. 

 They are generally supposed to be connected with reproduc- 

 tion ; but hitherto no precise observations have been made 

 upon them. In most, and perhaps in all the species of Nostoc, 

 many of these enlarged cells are scattered singly and de- 

 tached throughout the mucous matrix : they have doubtless 

 become separated from the filaments of which originally they 

 formed a link. 



If a Nostoc in the first period of its developement be ex- 

 amined, it will be observed to consist of a single moniliform 

 thread, short, and but little curved, immersed in a mucous 

 nidus. In each of the fully developed specimens of most of 

 the Nostocs, however, threads innumerable present themselves. 

 Now the question arises, in what way are those threads mul- 

 tiplied ? First, and chiefly I conceive, by the separation or dis- 

 location of the enlarged cells, whereby each filament is divided 

 into other shorter filaments ; and in the second place, probably 

 by the growth of those vesicles themselves ; but on this point 

 nothing positive is known. Independently of these two modes 

 of multiplication of the threads in each frond, no other con- 

 ceivable method exists. The filaments in every example of a 



